1. Field of the Invention
The present invention relates to a heat radiator, which is used to radiate heat being emitted from a power module, and the like, and a process for manufacturing the same. The power module is, for example, constituted by an electric element (or electric device), which serves as a heat generator.
2. Description of the Related Art
A chip (e.g., silicon chip, etc.), a power module, or the like, is essential to control a variety of apparatuses. In the chip, there are disposed electric elements with a high density on a semiconductor substrate. In a power module, there are disposed a large number of chips.
However, semiconductor products should usually be used in determined service temperature ranges. When they are used outside the ranges, they cause malfunctions. Accordingly, it is necessary to appropriately radiate heat, which is emitted from silicon chips, and so on. In particular, the higher the integration degree of chips is, or the more the control electric current enlarges, the more it is necessary to enhance the cooling ability for chips.
Hence, it has been carried out conventionally to dispose a radiator plate on the lower surface of silicon chips, etc. For example, in Japanese Unexamined Patent Publication (KOKAI) No. 11-126,870, there is disclosed a radiator plate, which comprises a metal-based composite material using a ceramics dispersant. Specifically speaking, the radiator plate comprises a metal-based composite material, in which a silicon carbide powder, serving as a ceramics dispersant, is dispersed in an aluminum alloy, serving as a matrix. The aluminum alloy, which exhibits a good heat transfer coefficient, is used as a matrix to secure a heat-radiating ability. The silicon carbide, which exhibits a small thermal expansion coefficient, is dispersed in the aluminum alloy to inhibit the radiator plate from warping. Note that fins are disposed on the heat-radiating-surface side of the radiator plate, and that they are manufactured by using a core, which is made from a readily-soluble salt (e.g., NaCl).
However, the radiator plate, set forth in the aforementioned publication, is formed as a uniform organizational (or compositional) construction as a whole from the heat-receiving surface, on which silicon chips, or the like, exist, to the heat-radiating surface, on which the fins exist. Consequently, because of the temperature gradient, which arises from the heat-receiving surface to the heat-radiating surface, a gradient also takes place in the thermal expansion of the radiator plate. Namely, the thermal expansion is large on the heat-receiving-surface side, and it is small on the heat-radiating-surface side. Thus, warpage takes place in the entire radiator plate. Therefore, on the heat-receiving-surface side of the radiator plate, there might arise the fears of coming-off of the silicon chips, etc., therefrom, the degradation of contacting ability and the deterioration of heat-radiating ability.
Further, since the silicon carbide is dispersed uniformly in the entire radiator plate, the thermal resistance enlarges to lower the heat transfer coefficient. Thus, the heat-radiating ability might be impaired.
Furthermore, the conventional radiator plate is manufactured by using the salt core. Note that, however, the salt core exhibits a thermal expansion coefficient of about 46xc3x9710xe2x88x926/K and the metal-based composite material exhibits a thermal expansion coefficient of about 8xc3x9710xe2x88x926/K. Accordingly, there might arise a large thermal expansion difference between them before and after the molten metal is solidified. Consequently, warpage might take place in the resulting radiator plate after the casting. Hence, the dimensions of the final product might not be stabilized.
Moreover, the salt core is manufactured for each of the radiator plates. In addition, it is necessary to wash away the salt core with water after casting the radiator plate. Hence, it is not possible to say that the manufacturing process, set forth in the aforementioned publication, is a preferable manufacturing process in terms of the man-hour requirement as well as the cost.
The present invention has been developed in view of these circumstances. It is therefore an object of the present invention to provide a radiator plate, which is good in terms of the heat-radiating ability, and which can inhibit the warpage adequately.
Moreover, it is another object of the present invention to provide a process for manufacturing such a radiator plate efficiently.
The inventors of the present invention researched earnestly to solve the problems, made trial and error to achieve the objects, and carried out a variety of systematic experiments repeatedly. As a result, they thought of dispersing a dispersant more on a heat-receiving-surface side of a radiator plate than on a heat-radiating-surface side thereof. Thus, they have completed the development of a radiator plate according to the present invention. At the same time, they have completed the development of a suitable process for manufacturing the present radiator plate.
A radiator plate according to the present invention can carry out the aforementioned object, and is characterized in that it comprises a metallic matrix exhibiting a predetermined coefficient of thermal expansion, and a dispersant being dispersed in the metallic matrix and exhibiting a coefficient of thermal expansion being smaller than that of the metallic matrix; that the radiator plate has a heat-receiving surface, on which an electric device serving as a heat generator is disposed, and a heat-radiating surface for radiating heat received from the heat-receiving surface; and that the dispersant is dispersed more on a side of the heat-receiving surface than on a side of the heat-radiating surface.
Since the dispersant of smaller thermal expansion coefficient is dispersed more on the side of the heat-receiving surface, in which an electric device serving as a heat generator is disposed, the thermal expansion is controlled on the heat-receiving-surface side. Accordingly, it is possible to secure the bonding ability or adhesion ability with respect to silicon chips, etc. Moreover, even when there arises a temperature gradient from the heat-receiving-surface side to the heat-radiating-surface side, it is possible to control or inhibit the warpage of the entire radiator plate because the dispersant of smaller thermal expansion coefficient is distributed more on the heat-receiving-surface side.
Moreover, contrary to a case where a member of small thermal coefficient is cast into a metal, in the heat radiator plate according to the present invention, it is possible to make the thermal resistance less and to inhibit a boundary layer from forming abruptly, because the dispersant of small thermal expansion coefficient is suitably distributed gradiently.
In particular, it is appropriate that, in the radiator plate according to the present invention, the metallic matrix can comprise aluminum as a major component and the dispersant can comprise a primary crystal including silicon as a major component.
The primary crystal (i.e., dispersant) comprising silicon as a major component exhibits a thermal expansion coefficient, which is in the proximity of a thermal expansion coefficient exhibited by a substrate made from silicon. Consequently, the thermal expansion difference can be furthermore diminished between the dispersant and the substrate. In addition, it is possible to readily produce the primary crystal comprising silicon as a major component, not by separately adding the dispersant to a molten alloy, but by controlling a solidifying temperature of the molten alloy. In addition, when the metallic matrix comprises aluminum as a major component, it is possible to obtain a radiator plate, which is good in terms of the thermal transfer ability and heat-radiating ability.
A process for producing a radiator plate according to the present invention can carry out the aforementioned object, and is characterized in that it comprises the steps of: pressurizing and charging a hypereutectic molten alloy into a cavity of a mold with a filtering member disposed therein, the filtering member having opposite sides, from one of the opposite sides of the filtering member at a temperature of generating a primary crystal or less; and solidifying the resulting molten alloy after accumulating the primary crystal, being generated in the pressurizing-and-charging step, on the one of the opposite sides of the filtering member.
By maintaining the hypereutectic molten alloy at an appropriate temperature, the hypereutectic component arises as a primary crystal. Then, the primary crystal, which arises in the cavity of the mold, is filtered out by the filtering member in the pressurizing-and-charging step, and is accumulated on the one of the opposite sides of the filtering member. Under the circumstance, when the resulting molten alloy is cooled by cooling the mold or by the other methods (i.e., the solidifying step), it is possible to obtain a radiator plate, in which the primary crystal is accumulated on the one of the opposite sides of the filtering member.
Moreover, it is appropriate that the hypereutectic molten alloy can be an aluminum-silicon molten alloy whose hypereutectic component is silicon.
Thus, it is possible to efficiently manufacture the aforementioned radiator plate. Note that the resulting radiator plate is constituted by the metallic matrix, which comprises aluminum as a major component, and the dispersant, which comprises a primary crystal including silicon as a major component.
In addition, in a case where the hypereutectic molten alloy is pressurized and charged from the heat-radiating-surface side with respect to the filtering member (see FIG. 1.), it is appropriate that the present production process can further comprise the step of removing the filtering member after the solidifying step.
Note that the filtering member, which comprises a formed substance, or the like, made, for example, from ceramic fibers, can remain on the resultant heat radiator plate. However, when the filtering member is removed, it is possible to obtain the heat radiator plate, which warps less, and which is good in terms of the heat-radiating ability.
In accordance with the radiator plate according to the present invention, since the dispersant of small thermal expansion coefficient is present more on the heat-receiving-surface side, it is possible to diminish the thermal resistance between the heat-receiving surface and the heat-radiating surface. Accordingly, it is possible for the present radiator plate to secure the heat-radiating ability. Moreover, the present radiator plate is inhibited from warping, and is good in terms of the dimensional stability as a final product.
In accordance with the process for manufacturing a radiator plate, it is possible to manufacture such a good radiator plate not only with a good productivity but also at a less expensive cost.